Why explicitly delete the constructor instead of making it private?

Question

When/why would I want to explicitly delete my constructor? Assuming the reason is to prevent its usage, why not just make it private?

class Foo {    public:      Foo() = delete;  }; 

Answer 1

How about:

//deleted constructor class Foo {    public:      Foo() = delete;        public:     static void foo(); };  void Foo::foo() {    Foo f;    //illegal } 

versus

//private constructor class Foo {    private:      Foo() {}        public:     static void foo(); };  void Foo::foo() {    Foo f;    //legal } 

They're basically different things. private tells you that only members of the class can call that method or access that variable (or friends of course). In this case, it's legal for a static method of that class (or any other member) to call a private constructor of a class. This doesn't hold for deleted constructors.

Sample here.

Answer 2

why explicitly delete the constructor?

Another reason:
I use delete when I want to assure that a class is called with an initializer. I consider it as a very elegant way to achieve this without runtime checks.

The C++ compiler does this check for you.

class Foo {    public:        Foo() = delete;        Foo(int bar) : m_bar(bar) {};    private:        int m_bar; } 

This - very simplified - code assures that there is no instantiation like this: Foo foo;